Plastics are susceptible to micro organisms. Flexible (plasticized) PVC and PUR foams are particularly more susceptible to attack by fungi, bacteria, mold and mildew, which leads to discoloration, odor and deterioration of mechanical properties. Plasticizers are a key food source for these microorganisms in PVC. Typical applications of flexible (plasticized) PVC and PUR foams include roof liners, tarpaulins, wall and floor coverings, pool liners, shower curtains, bath mats, outdoor furniture, insulation foams, and shoe soles as per Plastics Technology. A major trend is the use of biocides and antimicrobials in “hygienic” applications to arrest the spread of bacteria and diseases in kitchen and hospital equipment. Growth of antimicrobials (biocide) appears to be particularly strong in the medical/healthcare and consumer goods sectors. Hospital acquired infections are driving the use of biocides in health care products beyond medical devices. US Army & Marine Corps has issued specifications for inclusion of antimicrobials in military clothing and equipment. Some experts predict that consumer market will be a major market for biocides in future. In recent years, increasing public concern about food-borne pathogens and other germs in the environment has been driving consumer demand for antimicrobials in products made of polyolefins and other plastics ranging from houseware and toys.
Antimicrobial polymer additives can be fit into two broad categories as per Plastics Engineering : organic or inorganic. These systems have different attributes leading to different end-applications. Antimicrobial additives though referred to as biocides, have two different methods of effect: biocidal (killing the organism) and biostatic (preventing reproduction). Organic additives are biostatic, and inorganic additives combine biocidal and biostatic properties. Organic-based systems, which include the organometallic class of molecules, rely on small migratory molecules to introduce an antimicrobial effect at the polymer's surface. Once incorporated into plastic, they migrate, over time, out of the polymer matrix and onto the polymer surface, where an antimicrobial "film" is formed. Migration occurs as the molecules move down a concentration gradient out of the plastic. The migration is driven by the inherent compatibility differences between the organic antimicrobials and the polymer substrates in which they are dispersed. The resulting film on the polymers surface is replenished by additives within the substrate whenever the surface is wiped or washed, or when the additive is lost to the environment. This mode of action is beneficial because it can have a very high activity rate, and the migratory molecules can interact with large numbers of microbes very quickly. However, this affects the lifespan of activity, as the additives leach out over time. The addition rate and choice of organic additive are functions of the level of efficacy required and the duration of action needed. Commercially, organic technologies suit disposable items that have shorter lifespan than more durable and environmentally demanding products. Another consideration with organic systems is the effect of temperature during processing. As temperatures increase, organic molecules become more highly mobilized, resulting in excessive loss rates from the plastic. Also, organic antimicrobials often have thermal-decomposition temperatures similar to the temperature of the polymer-processing window. Polymers such as PVC and some low-temperature polyolefins are best suited to these additives. The environmental temperature in which finished products will be used can influence the migration rate and longevity of the active system. Lack of food-contact approval for these systems is a limitation, because of the mobility and solubility of these additives in food stimulants. The most common organometallic antimicrobials are arsenic-based materials such as oxybisphenox arsine (OBPA). While these additives are very efficient and cost-effective, their use has been limited because of environmental concerns and market perception regarding their long-term toxicity. These fears have unfortunately had a negative impact on market confidence despite OBPA being registered in USA by the Environmental Protection Agency (EPA). Resultantly, demand for non-arsenic-based formulations, such as the non-metal-containing isothiazalone family of biocides, and triclosan (chlorinated diphenyl ether) is growing rapidly. Arsenic-based formulations tend to offer cost advantages over their non-arsenic counterparts. Most suppliers of biocides for plastics use the active ingredients from chemical producers and then blend them at different concentrations with plasticizers, solvents, or resins. OBPA (oxybisphenoxarsine) is the most widely used active ingredient. Although newer biocide products tend to be based on other active ingredients, many formulators of biocides still regard OBPA as the most effective overall. OBPA is considered to be the largest anti microbial additive which is used in flexible PVC and has almost 67% share of the total anti microbial additive consumption in 2008.
Inorganic antimicrobials utilize metal ions as their active biocidal agent, and they remain in-situ once incorporated into the polymer matrix. The most commonly used metal ion is silver; others include copper and zinc. Silver ions are thought to disable bacterial cells by acting on them in multiple ways resulting in a strong biocidal effect. In the primary mode of attack, ions bind to the cell membrane, affecting its ability to regulate the diffusion and transport of molecules in and out of the cell. Similarly, once inside the cell, the ions target thiol groups on the proteins, which function as enzymes in their critical metabolic pathways. This denatures the enzymes, bringing about a loss of cell functional ability, which leads to cell death. The success of these systems relies on the delivery of minute quantities of ionic metal at the cell membrane. The metal ions are usually bound within a delivery system that stabilizes them, allowing their incorporation into the polymer, and then releases them through a process of ion exchange at the plastic's surface. The metal ions remain stored within the polymer and are continuously made available over the lifetime of the particular finished product. The additive-addition level and delivery mechanism regulate how quickly ions are released and the duration of the action. Some systems favour rapid release, such as wound-care applications, while others have a more controlled mode of action continuing over the lifespan of the substrate. Inorganic antimicrobial systems are nearly all silver based and the key types include colloidal silver, silver salts, silver zeolite/ion exchange resins, complex glasses containing metal ions, and nano silvers. They are gaining greater recognition, constrained only by their high cost. Cationic quaternary ammonium salt based anti microbial additive by Biosafe is a non toxic additive that is less costly compared to silver.
Unfortunately, as with any technology, there is a potential for overuse and this can lead to a build-up of potentially dangerous chemicals within the environment, along with much more resilient bacteria. However, the benefits they offer are important as careful use of these biocides protects vulnerable members of the community and considerably extends the lifetime of many of the materials in the built environment.
RTP Company, a global custom compounder of engineered thermoplastics has entered into an exclusive sales and manufacturing agreement with Biosafe Inc, to use their silane-based antimicrobial technology. The patented antimicrobial can be used by plastic processors as an additive masterbatch in a variety of molding and extrusion processes. The silane-based additive that allows plastics to inhibit microbic growth is more cost-effective than comparable silver-based antimicrobials without discoloration. "The active ingredient in these additives only needs to be at a 0.25-0.5% loading in the final product to be effective, making it more price competitive than traditional antimicrobials such as silver or triclosan. Unlike silver, which can cause yellowing or tarnishing, silane-based antimicrobials offer better colorability. An additional benefit of the Biosafe antimicrobial masterbatches is the low toxicity of the active biocide. It is non-migratory, maintain its effectiveness over time providing a sustainable means of prolonging the useful life of treated consumer and industrial goods. Silane-based antimicrobials contains no volatile organic compounds (VOCs), heavy metals, arsenic, or polychlorinated phenols. The active ingredient in Biosafe masterbatches complies with ISO 10993 tests for non-cytotoxicity.
Exilica has found a way of incorporating volatile organic compounds, such as fragrances, flavours and antimicrobials, into thermoplastics. The company claims that typical polymer processing temperatures cause organic compounds to evaporate. It has overcome this by shielding the compounds so that they survive two exposures to heat of 220 deg.C. The team uses u-Sq beads that act as microsponges and can absorb every metal of the periodic table as well as organics such as fragrances. The size, shape and robustness allow them to be mixed into polymers, acting as a delivery vehicle to carry chemical actives. So far the beads have been added to PP, polyester, nylon 6 and EPDM, with fragrances such as almond, rosehip and jojoba oil. The company has also managed to process the polymer into fibres to be woven. Markets include automotive or aerospace, in a cabin environment. The technique would be used to mask the smell of the polymer or introduce a fragrance into the environment. Another use is for tubing that needs to be antibacterial.
The European Food Safety Authority (EFSA) approved biocide 5-chloro-2-methyl-2H-isothiazol-3- one, mixture with 2-methyl-2H-isothiazol-3-one n food contact materials. The dossier was submitted by German company THOR GmbH. The body’s CEF Panel said the substance posed no safety concerns for the consumers as long as the “maximum residual amount in the finished products does not exceed 25 µg/dm² and the use of the mixture does not result in an anti-microbial effect at the surface of the polymer or on the food itself”.
A polymer that combines drug-eluting and self-cleansing agents could reduce the risk of bacterial infection through urinary catheters, say researchers at Queen's University Belfast. The material, derived from esters of acrylic and methacrylic acid, is melt-extruded in a confidential multilayer extrusion mechanism to produce a catheter surface that continuously delivers antimicrobial agents over prolonged periods, minimizing bacterial colonization. The concept is believed to be unique as the material has been designed to respond intelligently in vivo to minimize the likelihood of microbial biofilm formation and encrustation on the device surface. Should the prevention mechanism fail, a patent pending self-cleansing technology involves the production of self-shedding layers manufactured from pH-erodible polymers. During the process of infection, urea is broken down by urease-producing bacteria, producing ammonia that elevates urinary pH, triggering the surface-shedding of the material. The shredded material would be passed out with the urine so there is no issue with breakdown of materials.